Mix the sugar, cocoa, cornstarch, salt and coffee into a medium pot. Add the milk and mix well.
Over medium heat, heat the milk mixture, stirring very frequently.
In between stirs, set ramekins in a casserole pan or such - my 9x13 could hold six easily with room for ice - adding ice and water (and a little salt if you want to cool it even faster) about 1/2 -2/3 the way up the side of the ramekin.
When the pudding thickens at around boiling point, remove from the heat and mix in the butter and vanilla, and continue to mix until fully incorporated. Ladle pudding into the chilling ramekins, with additional ramekins or bowls on the side to capture the rest of the pudding.
Cover the pudding with plastic wrap, pressing right onto the surface to avoid a skin forming. Chill for at least 45 minutes. Will last in the fridge... I have no idea, we always eat it all within a day or two.